The big thing is a felt sense of security in one's salvation. As I shared, growing up in an Arminian tradition, I didn't have this sense of security. So my spiritual life always crackled with anxiety. A sin scared you, making you feel you'd stepped away from a state of grace. Consequently, you'd rush to pray for forgiveness as a means of "getting clean" again.
All of this creates a sneaky version of works-based righteousness.
Free will traditions will tell you that grace really is a gift, though you do have to "reach out and take it." This "acceptance" of the gift, the exercise of our free will in responding to the offer of grace, is what the Reformed tradition rejects. Because even a choice, that slight bit of human participation, can create room for human boasting. You can see the point. We pat ourselves on the back all the time for making good choices. And blame people for making bad choices. Praise and blame do get attached to our choices. So why wouldn't they do so when it comes to grace? Wanting to eliminate that possibility, the Reformed tradition nullifies the role of the human will in responding to grace. There is no choice, only God's election.
Again, while there are passages in the Bible that do talk about falling away from grace, I appreciate the desire to put a fence around grace, to make the whole of salvation the gratuitous and efficacious work of God. And beyond protecting grace at the initial moment of salvation, the "once saved, always saved" view also protects grace from how beliefs about "falling away" sneak in a works-based righteousness.
Specifically, while free will traditions argue that grace is really is a gift (that you can reject or refuse), they do struggle to keep grace protected throughout the lifespan. As I described above, if you can fall from grace then you have to put in a lot of work to maintain your status as saved. Constant moral vigilance and effort is demanded throughout your life. Simply put, while you might not need to work to get grace, you do need to work to keep it. And it's this effort in keeping grace that sneaks in a works-based righteousness. You're like a little duck swimming on the water. On the one hand, you're saved. You're not drowning. But underneath your little feet are furiously paddling to keep yourself afloat.
The point here is that free will traditions can say, I think legitimately, that the offer of grace is a gift even if you have to accept in an act of will. However, the subsequent possibility of falling from grace sneaks in a works-based dynamic after the fact due to all the effort you have to go through to keep grace secured. That is to say, "once saved, always saved," the doctrine of the perseverance of the saints, isn't just about protecting grace at the start of the journey, by eliminating the human will from the equation. "Once saved, always saved" is also trying to protect grace across the lifespan, cutting off anxious effort to prevent "falling" from grace. The fear of falling creates worried human effort. We might not work to "accept" grace in an act of free will, but our will and effort is most definitely involved in keeping grace.
In short, grace doesn't feel much like grace if it is a fragile, precarious thing.
